Actually that's not true. There can even be cases where upgrades are available for each segment but a through traveler booked as a direct flight cannot upgrade because the same first-class seat is not available. For example if 3B, 4B, 5B and 6B are the only open F seats on the first segment and that segment is X1+ and 3E, 4E, 5E and 6E are the only open F seats for the second segment which is also X1+, an upgrade will not be available for a through passenger.